Camera modules used in motor vehicle operation are not only subject to high mechanical stresses, but to considerable temperature fluctuations as well, and to the effects of moisture. When a camera module is used that does not have a completely impervious design, under the climatic conditions typical of automotive applications, this can lead, on the one hand, to dew formation in the optical path and, on the other hand, to degradation of the image quality. Moisture condensation is often observed when a high relative air humidity has set in inside of the camera module for a length of time at elevated temperatures, and the camera module is then cooled within a short period of time.
Optical recording units constructed from a compound lens and a sensor chip are typically designed to be hermetically sealed. The ingress of moisture resulting from vapor permeability that may be excessive for many of the materials used, can be controlled by using absorbing agents, such as zeolite, for example. Besides gluing the front lens element of a lens stack in place, it is conceivable to use sealing rings, thus, for example, O-ring seals of various materials. This is a practical approach for achieving an impervious design. Compared to fixing in place by gluing, sealing rings made of elastomer material, for example, advantageously make a reconditioning possible. Moreover, when sealing rings are used, there is no risk of contamination. To reliably ensure the sealing action when a sealing ring is used, a defined compression of the sealing ring is produced by the engagement of the threaded connection. Because manufacturing variances are inherent in the structural heights within a lens stack which may include a plurality of lenses, a constant compression is not ensured for all pieces when a sealing ring is clamped between the frontmost lens surface and a contact mass, such as a ring-shaped area in the tubular housing of the compound lens. Moreover, it must be ensured that the sealing ring is not unevenly clamped, which can result, for example, from localized overstraining when the parts are screwed in place during assembly. To ensure the requisite image quality for compound lenses in the application cases mentioned at the outset, it is important to have as few deviations as possible in the positioning of the individual lenses. Thus, the individual lenses of a lens stack require appropriate guidance; the guidance of the individual lenses must not be adversely affected by skewed sealing rings. When a seal rests on a front surface, any encroachment on the available beam diameter is disadvantageous, since the edge thickness of the front lens cannot be arbitrarily reduced.
A waterproof camera is described in Japanese Patent Application No. JP 2002 090603. The waterproof camera is equipped with a camera housing which has a front part and a rear part. A lens stack is accommodated inside of the camera housing. Within the lens stack, a hollow space is formed behind the front lens. A lens suited for taking wide-angle shots is placed inside of this hollow space. The front lens of the lens stack is provided at the opening of the lens stack and, in addition, is used as a seal. A projection at the edge of the lens stack is caulked onto the lens side in a thermal joining process. An O-ring seal is provided between the outer periphery of the lens and the peripheral surface of an inner wall for the lens stack to seal off the annular gap therebetween. In addition, another O-ring seal is provided between the outer peripheral surface of the edge part of the lens stack and the inner peripheral surface of the edge part of the front camera housing, in order to seal the clearance space therebetween.
From U.S. Pat. No. 5,519,543, an optical system for an inspection device is known. A camera that can be lowered into a bore hole in the earth or into a pipe laid in the earth, includes a tubular body made of thermally insulating material. A front lens group and a rear lens group are disposed at mutually opposing ends of the tubular member. The mutually opposing lens groups at the front and rear ends are thermally isolated from one another, a vacuum chamber, which essentially extends along the optical pathway of the light, being formed in the tubular body.